"Tornado Alley," which includes many of the Great Plains states and parts of Texas, is the the most notorious staging ground for U.S. twisters. But in December tornadoes tend to form in the southeast and east Texas, fueled by the warm, moist air coming off of the Gulf of Mexico.

A massive tornado passes through Rowlett, Texas, on December 26, 2015. The intermittent flashes of light are caused by lightning and downed power lines.

Tornado Science

Scientists don’t completely understand how tornadoes like this weekend’s form. And meteorologists struggle with forecasting tornadoes, since they’re short-lived, finicky, and relatively tiny compared with other atmospheric phenomena.

Here's what we do know: A tornado is a violently rotating column of air that extends between the Earth's surface and a cloud.

Those ingredients include warm moisture near the surface and relatively cold, dry air above. But supercells require winds that increase in strength and change direction with height, generating horizontal tubes of rotating air parallel to the earth’s surface.

Warm air below a horizontal vortex rises “like a hot-air balloon," says Brooks, and bends it into an arch. One of the two vertical vortices forming the arch then peters out. Shearing winds tilt the top of the surviving vortex, causing cool rain to fall away from the warm surface air fueling the newly born mesocyclone, letting the supercell purr like a finely tuned engine.(See an animation of how tornadoes form.)

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Powerful Tornadoes are Rare

Most tornadoes last for fewer than ten minutes and travel only a few miles before they dissipate. But large tornadoes can last up to 30 minutes and can have wind speeds of more than 300 miles (483 kilometers) per hour, which can rip buildings off their foundations.

What defines powerful? In many countries, including the United States, tornadoes are measured with the Fujita scale or the updated Enhanced Fujita Scale. An F0 or EF0 tornado damages trees but substantial structures are left unharmed; a tornado in the strongest category—F5 or EF5 —blows away buildings.